The port size depends on what valve you are running. My tank has all 1/2" ports, so I just used bushings to step it down for the 1/4" compressor hose, 3/8" drain plug, 1/4" pressure switch.
The common setup seems to be 4 ports on the side for valve, a port on each end for compressors, a port on the bottom for drain, and a port somewhere for the pressure switch.
A 5 gallon tank is good, any bigger and you will need 2 compressors. I have a 7 gallon tank and it's way too much for my single compressor now. I don't fill the tank, so it's not too bad.
If you have 2 ViAir 380 compressors (a nice setup), you can run a 7 gallon tank, or even do dual tanks. It all comes down to what you want to run and where it fits.
Here is a shot of mine mounted between the frame rails. The driver side is plugged, the drain on the bottom, compressor feed on the passenger side and the pressure switch on the back.
